Since an electrophotographic device involves an advantage in that the printing speed thereof is high to give a high printing quality, it has widely been applied to various fields of duplicators and laser beam printers.
As an electrophotographic photoreceptor used in such an electrophotographic device, inorganic photoconductive materials such as selenium, selenium-tellurium alloys, selenium-arsenic alloys, cadmium sulfide and zinc oxide have widely been known. Organic photoconductive materials have come to be used to form an organic photoreceptor. For instance, development of charge generating materials such as polyvinyl carbazole compounds as well as bisazo compounds, perylene pigments and phthalocyanine pigments and low molecular weight charge transporting materials such as pyrazoline compounds and hydrazone compounds has been promoted and use of them is being popularized gradually. Regarding the constitution of the photoreceptor using organic photoconductive materials, a function-separated laminate structure composed of a charge generating layer containing Se/polyvinyl carbazole or the like and a charge transporting layer as separated from each other has been developed and has come to be utilized more popularly than the simple structure comprising a polyvinyl carbazoletrinitrofluorenone charge transfer complex or the single layer structure containing a charge generating material as dispersed in a binder resin, which have been developed previously. Thus, the capacity of the photoreceptor has been improved.
In general, a photoreceptor is continuously exposed to mechanical abrasion in an electrophotographic apparatus due to contact with the developing device, developing agent and cleaning device. Since the organic photoreceptor is mainly composed of organic materials, it involves a drawback that the mechanical strength is low. Thus, the thickness of the light-sensitive layer decreases or a toner would adhere onto the scratches as formed on the surface of the light-sensitive layer during the electrophotographic operation and, as a result, the scratches on the light-sensitive layer are printed as image defects or fog generates on white non-image areas in the printed images.
In order to overcome the drawback, an attempt has been made to improve the surface of the light-sensitive layer of a photoreceptor and to reduce the abrasion or scratches of the light-sensitive layer thereof so as to prolong the life of the photoreceptor itself.
For instance, various reports have heretofore been made of adding an abrasion-resistant fine granular material (hereinafter sometimes referred as "fine grains") to the surface layer of a light-sensitive layer of a photoreceptor so as to reduce abrasion of the light-sensitive layer and to reduce scratches on the surface of the layer. JP-A-2-2072 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a method of adding spherical powder of a hardening type resin to the surface layer of a photoreceptor, in which use of spherical silicone resins and the like is exemplified. JP-A-63-244039 discloses addition of a solid lubricant such as a metal soap; JP-A-1-205171 discloses addition of an inorganic filler; and JP-A-2-143257 discloses addition of surface-treated polyethylene powder.
However, if fine grains are added to the light-sensitive layer of a photoreceptor, the light as applied thereto is generally scattered by the added fine grains to lower the sensitivity of the photoreceptor or to retard the migration of the generated carrier, whereby the light attenuating characteristic of the photoreceptor is deteriorated to yield a high residual potential. As a result, addition of such fine grains causes various troubles of generation of fog, black peppers and dot-like non-image areas in the image formed.
Further, addition of such fine grains also causes still other various problems, depending upon the kind of materials for them. For example, fine grains of an inorganic material have a rapid precipitating rate in a coating composition so that the coating composition containing them involves a problem of stability. Conventional organic fine grains are insufficient with respect to the heat resistance and solvent resistance. Where electroconductive fine grains are added to a charge transporting layer, they noticeably lower the charged electric potential of the photoreceptor.
JP-A-63-2072 and JP-A-2-240659 disclose addition of a fine grains of a spherical melamine resin and fine grains of a benzoguanamine-formaldehyde condensate, respectively, to the light-sensitive layer of a photoreceptor.
In the case of adding fine grains of a melamine resin, the grains display an excellent effect for improving the abrasion resistance of the light-sensitive layer since the fine grains of a melamine resin are extremely hard and tough. However, since the grains have a high water-absorbing capacity so that when they are added to a photoreceptor, it causes potential trapping in the photoreceptor. As a result, the residual potential of the photoreceptor is noticeably elevated due to the addition of the powder so that the potential fluctuation under environmental variation becomes great. Thus, addition of the grains involves such various problems.
In the case of adding fine grains of a benzoguanamine-formaldehyde condensate to the light-sensitive layer of a photoreceptor, since the grains are coarse to have a large grain size of an order of 10 .mu.m. If such coarse grains are added to such a charge transporting layer having the largest thickness of from 10 to 40 .mu.m, the surface roughness of the surface layer of the photoreceptor would be great so that the uniformity of the latent image to be formed would be lost. As a result, poor images having much fog could only be obtained, defectively, by the use of the photoreceptor containing the grains.
As mentioned above, the prior art of improving the abrasion resistance of a photoreceptor still involves various problems.
The present invention has been made so as to overcome the problems in the prior art, in consideration of the current situation mentioned above.